The present invention relates to an intensity correction device for detecting an intensity of an input image and automatically adjusting the intensity of a video signal from a television receiver, a video tape recorder, a video projector or so forth, for obtaining an optimal image.
In recent years, there is a tendency for requiring higher picture quality for television receivers. For this, an intensity correction device which can automatically obtain an optimal intensity with detecting feature of an input image has been given higher importance.
Description of a conventional intensity correction device will be given with reference to FIGS. 8 and 9. FIG. 8 shows construction of the conventional intensity correction device. In FIG. 8, reference numeral 1 denotes a histogram memory for obtaining luminance distribution of an input luminance signal. Reference numeral 2 denotes a look-up table operating circuit for performing accumulation of a histogram and normalizing respective data so that the maximum cumulative frequency becomes coincident with a maximum value of an output luminance signal. Reference numeral 3 denotes a look-up table memory for storing data normalized by the look-up table operating circuit 2 and for permitting output of a correction signal corresponding to a luminance level of the input signal.
Operation of the intensity correction device constructed set forth above will be discussed below with reference to FIG. 9.
FIG. 9 is a plurality of graphs illustrating a manner of luminance conversion in the prior art.
At first, a histogram is established by accumulating input luminance levels into appropriate intensity groups. Then, the luminance distribution of an input luminance signal a, as illustrated in FIG, 9A, is stored in the histogram memory 1. The contents of memory 1 is periodically cleared to reset respective data to zero. The interval for clearing the data in memory 1 is typically selected to be a period corresponding to a vertical scanning interval of the video signal of the vertical scanning interval. Next, look-up table operating circuit 2 accumulates the data of the histogram, calculates a normalizing coefficient so that the maximum cumulative frequency of occurrence of the input luminance can be the maximum value of the output luminance level, and multiples respective data of the histogram by derived coefficient. The results are stored in look-up table memory 3. This procedure is illustrated in FIGS. 9B and 9C. Look-up table memory 3 outputs a difference between the data corresponding to a luminance level of the input signal a and the luminance level as a correction signal d as shown in FIG. 9D. Intensity correction is performed with this correction signal d.
However, in the construction set forth above, when the input luminance signal is concentrated at a particular luminance level, the luminance level is corrected in an excessive manner as illustrated in FIGS. 9E to 9H. In the case of a dynamic image of the television, the corrected image is far different from the actual image and the picture quality is rather degraded.